This invention relates generally to computed tomography (CT) imaging and more particularly, to filtration of an x-ray beam in an imaging system.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
The x-ray source is typically comprised of an evacuated glass x-ray tube containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by means of a high voltage across the anode and cathode.
The spectrum of the x-rays produced encompasses a band of radiation of differential frequencies having different energies. The short wavelength radiation of higher energy is referred to as "hard" x-ray radiation and the longer wavelength radiation of lower radiation is referred to as "soft" x-ray radiation. The very lowest energy x-rays are almost entirely absorbed by the body and therefore provide little contribution to the x-ray image. Nevertheless, these soft x-rays contribute to the total exposure of the patient to harmful ionizing radiation.
In at least one known CT system, a filter is used to remove or reduce the amount of "soft" x-rays. Filters are typically of a "fixed" type or a "shaped" type. The fixed filters are used to improve beam quality by removing soft x-rays which contribute to patient dose but do not contribute to image data measurement. Shaped filters are used to modify the x-ray intensity as a function of fan angle to obtain a more uniform x-ray intensity when a patient is present. The shaped filters are used to reduce x-ray intensity toward a patient extremity where less x-ray beam penetration is required. However, as a result of the different types and areas of the body to be scanned, selection of the ideal filter is difficult, if not impossible. As a result, the selected filter typically compromises either patient dose or beam quality.
Accordingly, it would be desirable to provide a filter which allows selection of filtration characteristics depending upon the scan to be completed. More specifically, the filter may be selectably configured to provide proper filtration for suitable x-ray beam quality and intensity for various types of scans.